1. Technical Field
This disclosure relates generally to gas fired combustion apparatuses such as residential and light commercial furnaces or heaters. More specifically, this disclosure relates to a combustion system for use in such a gas fired apparatus characterized by a reduced level of emission of oxides of nitrogen (NOx) that are obtained, at least in part, by premixing the fuel and air prior to ignition. Still more specifically, this disclosure relates to a low-pressure drop, premixed fuel/air, induced draft gas burner with an inlet or upstream geometry that stabilizes the system and produces a stable flame.
2. Description of the Related Art
The combustion natural gas, liquefied natural gas and propane forms NO with other combustion products. Because these fuels contain little or no fuel-bound nitrogen per se, oxygen and nitrogen in the air that react at the high combustion temperatures are responsible for the formation of NOx. Governmental agencies have passed legislation regulating NOx emissions by gas furnaces and other devices. For example, in certain areas of the United States, e.g., California, regulations limit the permissible emission of NOx from residential furnaces to 40 ng/J (nanograms/Joule of useful heat generated). Future regulations will most likely restrict NOx emissions from residential furnaces from 40 to less than 15 ng/J.
Current gas furnaces often use a particular type of gas burner commonly referred to as an in-shot burner or two-stage burner. Such burners include a burner nozzle having an inlet at one end for receiving separate fuel and primary air streams and an outlet at the other end through which mixed fuel and primary air discharges from the burner towards the heat exchanger. Fuel gas under pressure passes through a central port disposed at or upstream of the inlet of the burner. The diameter of the inlet to the burner is larger than the diameter of the fuel inlet to form an annular area through which atmospheric air (a.k.a. primary air) enters the burner nozzle with the incoming fuel gas. The burner may include a straight or arcuate tube having an inlet section, an outlet section and a transition section disposed therebetween, which is commonly referred to as a Venturi section.
The primary air mixes with the fuel gas as it passes through the tubular section of the burner to form a primary air/gas mix. This primary air/gas mix discharges from the burner and ignites as it exits the nozzle outlet section forming a flame projecting downstream from a flame front located immediately downstream of the burner outlet and in front of a heat exchanger inlet. An inducer fan draws secondary airflow into the burning mixture downstream of the burner and into the heat exchanger with the combusting gases in order to provide additional air to support combustion.
In order to comply with current and future NOx regulations, new burner designs will replace the current in-shot burner designs. The new burner designs will premix the air and fuel before combustion, without the aid of secondary air. The new premix burner designs are coupled to the heat exchanger inlet instead of providing a gap between the burner and heat exchanger, which allows for the entrainment of secondary air. By eliminating the use of secondary air, the premix burners control the premixing of the fuel and air and provide a lean mixture for combustion, which produces less NOx than traditional in-shot burners.
One problem associated with such premix burner designs is noise caused by pressure fluctuations. Pressure fluctuations in a fuel nozzle may cause fuel flow-rate fluctuations. Fuel flow-rate fluctuations may interact with the burner flame to produce pressure oscillations. The resulting fluctuation cycles may lead to oscillations with relatively large amplitude depending upon the magnitude and phase of the interactions. In short, pressure fluctuations lead to flame instability, which leads to undesirable noise.
Although feedback analysis is known to those of ordinary skill in the art of combustion dynamics, what is still needed are systems and methods that apply feedback analysis in the context of induced draft heating devices such as residential gas furnaces and other practical applications without interfering with efforts to reduce NOx and/or CO emissions.